FIG. 1 shows a conventional lift truck 10 whose basic function of lifting and transporting cargo is well-known. Drive wheels 12 are connected to a drive axle (not shown) and steer wheels 14 are connected to a steer axle 20 shown in FIG. 2. The drive axle is rigidly connected to the frame of lift truck 10. The steer axle 20 is used for steering the truck 10 and includes a suspension system shown in more detail in FIG. 2.
FIG. 2 is a top-view of the steer axle 20 used in the lift truck 10 shown in FIG. 1. The rear wheels 14 are attached to opposite ends of the steer axle 20. Stubs 20A and 20B extend from the opposite back and front ends, respectively, of the steer axle 20. A pair of brackets 22A and 22B are bolted to the truck frame 24 and hold the stubs 20A and 20B, respectively.
FIG. 3 is a rear sectional view of the steer axle 20. From this view, only bracket 22A is shown. Bracket 22B is similar to bracket 22A. The stub 20A of the steer axle 20 is centered about a center point 21. The stub 20A is held in substantially the same relative position within the bracket 22A about center point 21 by a rubber bushing 30. The rubber bushing 30 allows the steer axle 20 to pivot about the center point 21 when the steer axle 20 articulates (laterally inclines) either clockwise or counter clockwise.
Two articulation stops 32A and 32B are located on the upper surface of the steer axle 20. The spaces between the articulation stops 32A and 32B and the truck frame 24 are referred to as articulation gaps 34A and 34B, respectively. When the lift truck 10 is at rest, or traveling in a straight line on level terrain 31, there is little articulation of the steer axle 20 and the articulation gaps 34A and 34B remain relatively constant.
As long as the articulation stops 32A and 32B do not contact frame 24, the steer axle 20 is free to pivot about center point 21 independently of the frame 24. This pivoting of steer axle 20 allows the lift truck 10 to maneuver over uneven terrain and obstacles, or make turns, without effecting the lateral displacement of the frame 24.
The size of the articulation gaps 34A and 34B determine how far the steer axle 20 can articulate without laterally displacing the frame 24. If the steer axle 20 articulates far enough on one side, one of the articulation stops 32A or 32B contacts frame 24. In this articulation stop contact position, any further lateral articulation of the steer axle 20 equally articulates the frame 24.
Larger articulation gaps 34A and 34B can increase how much the steer axle 20 can articulate before one of the articulation stops 32A and 32B contacts frame 24. Larger articulation gaps 34A and 34B allow more articulation of the steer axle 20 without laterally displacing the frame 24.
FIG. 4 shows stability profiles for the suspension system shown in FIGS. 2 and 3. The lift truck 10 has a triangular stability profile RSU when the articulation stops 32A and 32B (FIG. 3) are not contacting the truck frame 24. With the triangular stability profile RSU, the lift truck frame 24 is supported at the R and S locations of drive tires 12 and at a U location along the centerline of the steer axle 20.
The lift truck 10 changes to more of a rectangular shaped stability profile RSTV when either of the articulation stops 32A or 32B come in contact with the frame 24. When the steer axle 20 pivots sufficiently to contact either one of the articulation stops 32A or 32B (FIG. 3), the steer axle 20 moves into a rigid non-pivoting relationship with the frame 24. This moves the lateral support locations for the rear end of frame 24 from centerline location U out to the T and V locations at the rear wheels 14.